1. Field of the Invention
The present invention relates to a surface inspection method and a surface inspection apparatus that are intended for detecting micro contaminant particles and defects present on a semiconductor substrate (a semiconductor wafer).
2. Background Art
In production lines of semiconductor substrates (semiconductor wafers), contaminant particles adhering to the substrate surface and defects, such as scratches occurring during processing, are inspected in order to monitor the dust generating condition of the manufacturing equipment. For example, in a semiconductor substrate before the formation of circuit patters, it is necessary to detect micro contaminant particles and defects on the surface to the nearest several tens of nanometers or less. In the above-described inspection of the substrate surface, crystal defects present in a shallow region near the substrate surface and the surface roughness of the substrate surface also become objects to be inspected in addition to the above-described contaminant particles and defects.
As a conventional technique for detecting microdefects on the surface of an objected to be inspected, such as a semiconductor substrate, for examples, as described in U.S. Pat. No. 5,798,829, there has been available an inspection technique that involves forming an illumination spot of a predetermined size by the fixed irradiation of focused laser beams onto the surface of a semiconductor substrate, detecting scattered light from a contaminant particle that is generated in the case of the presence of a contaminant particle adhering to the semiconductor substrate, when the contaminant particle passes through this illumination spot, and inspecting contaminant particles and defects on the whole surface of the semiconductor substrate. In this case, even when a contaminant particle and a defect do not pass through the illumination spot, in the above-described illumination spot, scattered light (hereinafter referred to as background scattered light) is constantly generated due to micro surface roughness (microroughness) on the semiconductor wafer. It is known that in the detection of micro contaminant particles, shot noise deriving from the above-described background scattered light is generally predominant in the noise components of an inspection signal. Because this shot noise is proportional to the square root of the intensity of the light from which the shot noise derives, the noise level in the detection of a micro contaminant particle increases roughly in proportion to the square root of the intensity of background scattered light.
On the other hand, in the case of a micro contaminant particle to which the law of Rayleigh scattering can be applied, when a surface of a semiconductor wafer is irradiated with p-polarization from such a low elevation angle as the Brewster angle to a silicon crystal, it is known that the scattered light from the contaminant particle does not have strong directivity in the direction of azimuthal angle and scattering occurs with almost the same intensity in the directions of all azimuthal angles. In the case of a semiconductor wafer that is polished well, in general, background scattered light deriving from surface roughness does not exhibit extremely strong directivity in the direction of azimuthal angle.
In this case, therefore, from the standpoint of ensuring the S/N ratio of detection signals, it is desirable to detect scattered light diffused in the directions of all azimuthal angles by uniformly focusing the light. In the photodetector of the above-described conventional technique, which collectively receives all of scattered light in the direction of azimuthal angle, a desirable S/N ratio can be obtained.
However, the background scattered light deriving from the surface roughness (microroughness) of a semiconductor surface may sometimes have strong directivity. For example, it is known that in an epitaxial wafer and the like, the background scattered light deriving from the surface roughness depending on a relative relationship between crystal orientation and the direction of illumination may sometimes have strong directivity. In such a case, as described above, a larger noise component is contained in an output signal of a photodetector that performs detection at an azimuthal angle at which the background scattered light deriving from surface roughness is strong. For this reason, it is not advisable that a scattered light signal detected at an azimuthal angle at which the background scattered light deriving from surface roughness is strong and a scattered light signal detected at an azimuthal angle at which the background scattered light deriving from surface roughness is weak are equally treated.
On the other hand, U.S. Pat. No. 7,002,677, which describes a technique for partially cutting off scattered light that travels in a specific direction, claims that it is possible to improve the S/N ratio of scattered light signals from a contaminant particle/defect, which is the object of inspection, by performing control to ensure that a photodetector partially shields the direction of azimuthal angle in which background scattered light is strong and receives scattered light only in the direction of azimuthal angle in which background scattered light is weak.
In the technique described in U.S. Pat. No. 7,002,677, a programmable light selection array is disposed on an optical path between a scattering object and a photodetector, and an azimuthal angle at which background scattered light is strong is partially shielded by controlling this array. In this method, the scattered light in the direction of each azimuthal angle is controlled by “on/off control” in the manner of a selection between the two: “light guiding/light cutting-off” i.e. “using/not using”. This applies also to another technique that is similar to the above technique. In this technique, partial cutting-off of scattered light is not performed; instead, among a plurality of photodetectors disposed in a plurality of angular directions, only output signals of a photodetector disposed in the angular direction in which background scattered light is weak are used and output signals of a photodetector disposed in the angular direction in which background scattered light is strong are not used. In these techniques, the scattered light that travels in the angular direction in which output signals from a photodetector are not used, is not received by the photodetector. Therefore, among the light signals included in the scattered light that is not received, the light signal components from a contaminant particle and a defect that are to be detected are discarded without being used. This is valid for a case where “not less than 99.9% of the total quantity of scattered light from a surface of an object to be inspected is background scattered light” as described in U.S. Pat. No. 7,002,677 and does not pose any problem in this case.
However, this poses a problem when the relative ratio of background scattered light to the total quantity of scattered light and the anisotropy of background scattered light in angular directions are not very large as described above. Now think of, for example, a scattered light detection system in which four photodetectors are arranged in four angular directions. If it is assumed that output signals of each photodetector consist of “an aimed signal deriving from the scattered light from a contaminant particle and a defect+a background signal deriving from background scattered light+noise” and that the principal component of the noise is shot noise deriving from background scattered light (this assumption is realistic in many cases), then the noise becomes proportional to the square root of the background signal deriving from background scattered light. For example, it is assumed that the compositions of output signals of the four photodetectors are as shown in “Detector #1” to “Detector #4” of Table 1.
TABLE 1BackgroundsignalAimed signal derivingderiving fromfrom the scattered lightbackgroundfrom a contaminantscatteredS/Nparticle/defectlightNoiseratioDetector #11.0001.0001.0001.000Detector #21.0002.0001.4140.707Detector #31.0001.0001.0001.000Detector #41.5004.0002.0000.750Even addition of4.5008.0002.8281.591#1 to #4Addition of only2.0002.0001.4141.414#1 and #3
At this time, in the two cases: (1) a case where the output signals of the four photodetectors are evenly added and (2) a case where the output signals of photodetectors #1 and #3 are evenly added by not using the output signals of photodetectors #2 and #4 (or by cutting off light so that scattered light does not become incident on photodetectors #2 and #4), the S/N ratio (the ratio of an aimed signal to noise) after addition is as shown in “Even addition of #1 to #4” of Table 1. On the assumption that the noise in each of the photodetectors is statistically independent, synthesized noise is found as the square root of the residual sum of squares of each noise. As is apparent from the results of Table 1, it is apparent that when the relative ratio of background scattered light to the total quantity of scattered light is not very large as described above, there are cases where the method that involves “cutting off light in an angular direction in which background scattered light is strong/not using” is inferior to the method that involves evenly detecting scattered light in all angular directions” in terms of S/N ratio.
The present invention has been made in view of the above circumstances, and provides a surface inspection method and a surface inspection apparatus that are capable of detecting scattered light from a contaminant particle and a defect at a good S/N even when the relative ratio of background scattered light to the total quantity of scattered light and the anisotropy of background scattered light in angular directions is not relatively large in a case where background scattered light deriving from the surface roughness of a semiconductor wafer has directivity in the direction of elevation angles or azimuthal angles and in a case where the directivity of background scattered light changes depending on positions on a wafer to be inspected.